Ball mill grates



Aug. 30, 1960 J. P. WALES BALL MILL GRATES 2 Sheets-Sheet 1 Filed Sept. 19, 1955 INVENTOR.

Je ss P. Wales Tig.-Z

ATTORNEYS Aug. 30, 1960 J. P. WALES 2,950,869

BALL MILL GRATES 2 Sheets-Sheet 2 Filed Sept. 19, 1955 INVENTOR.

Jess P. Wales F i .-5 BY ATTORNEYS BALL MILL GRATES Jess P. Wales, Denver, Colo., assignor to Mine and Smelter Supply Company, Denver, Colo., a corporation of Colorado Filed Sept. 19, 1955, Ser. No. 535,161 3 Claims. (Cl. 241-70) This invention relates to grinding equipment and more particularly to non-plugging discharge grates for ball mills and like equipment.

Ball mills for grinding or pulverizing material are being widely used throughout the world and are generally of the grate discharge type or overflow type. Many wet grinding overflow type ball mills have been converted to the grate discharge type. These conversions have provided a substantial increase in tonnage throughout in the mills ranging from an increase of ten to forty percent grinding capacity with from five hundred to seven hundred percent circulating load.

In the early stages of the low pulp line discharge grate type of ball mills the practice was to finish grind to the desired fineness in one pass of the material through the mill, and the grates had narrow slots or openings usually 7 or /4" wide. These narrow slots did not present a plugging problem.

The present practice of operation is in closed circuit with a sizing classifier. The material passing through the grate is discharged by means of litters and a center discharge cone through the trunnion of the mill into the classifier where the desired mesh product is overflowed and the sands or oversize particles are returned from the classifier to the mill where it is mixed with new incoming feed for another circulation through the grinding zone for still further grinding. This creates the circulating load which generally runs from 4:1 up to, in some cases, as high as 15:1 depending on the hardness of the ore or material, size of original feed, final mesh product required, and the tonnage desired.

Larger ball mills are now being used and single units are grinding as high as 75 to 80 tons of new feed plus circulating loads of 4:1 and more each hour and these large tonnages and circulating loads have necessitated the use of larger slot openings in the discharge grates. Slot opening will vary with the different grinding operations from /2" width of slot to 1" slot width. Merely providing the larger slots, however, has caused serious difliculties in many installations due to the slot openings becoming plugged or choked with small balls and tramp steel. The plugging usually begins in the slots at the periphery of the grate and then extends toward the center of the mill. The discharge area through the grate may be up to sixty to seventy percent closed through plugging, and the efiiciency is greatly reduced because the mill ceases to operate on the low pulp level principle. Since the plugging is from the periphery toward the center, the pulp level rises and the mill operates as a semi-overflow mill.

Ball mill grates of the prior art are designed with radial or tangential slots or a combination of both, and the slots are made with'passages through the metal of the grate at right angles to the wearing face of the grate. Small balls and tramp steel wedge into the grate openings and the severe action of the ball load at the toe of the ball path increases the wedging action. The cascading balls Wear off the protruding portions so that the wedged par atent ice ticles become flush with the wearing face of the grate, making the particle difiicult to remove.

According to the present invention, I have provided a grate which is substantially non-plugging or choking and which accelerates the flow of pulp through the The slot passages through the grates are placed at an angle to the wearing face so as to prevent a straight, axially parallel discharge of pulp through the grates. The angle of the passages is directed from an opening in the wearing face to an opening closer to the center of the mill diameter on the discharge face thereof.- The slots may, also, be placed at an angle from the tangent to further decrease plugging possibilities. The angle from the tangent varies according to the ball load and mill speed, and it may vary from zero to about thirty-five degrees to obtain the maximum ball action across the entire length of the slots. The parabolic path of the balls substantially determines the angle from the tangent for maximum efliciency. The direction of rotation of the mill whether clockwise or counterclockwise, must be considered when the slots are placed at an angle from the tagent. While the straight tangential slot with a 30 angle through the grate will operate in either direction of rotation, maximum efiiciency may be obtained by sloping the slots up to about 35.

Included among the objects and advantages of the present invention isto provide a substantially nonplugging grate for ball mills which provides an increased flow of pulp through the grates. The grate includes a passage which extends through the metal of the grate at an angle to the wearing face of the grate, and in one embodiment the lateral extent of the slot is placed at an angle to the tangent corresponding to the ball load and the mill speed. The'slope of the lateral extent of the slot from the tangent is arranged to be relatively positioned to the rotation of the mill so that the slot at about the horizontal center line of the mill will be substantially in parallel alignment with the parabolic path of the balls in the mill.

These and other objects and advantages of the invention will be readily ascertained by referring to the following description and illustrations in which:

Fig. 1 is a partial sectional view of a ball mill illustratng the positioning of a grate according to the invention;

Fig. 2 is a front elevational view of a ball mill grate according to the invention;

Fig. 3 is a front elevational view of a section of a ball mill grate illustrating the slot arrangement of the invention;

Fig. 4 is a cross sectional view of the grate of Fig. 3 taken along line 44;

Fig. 5 is a front elevation of a modified ball mill grate according to the invention; and

Fig. 6 is a cross sectional view of the grate section of Fig. 5 taken along line 6- 6.

The ball mill in Fig. 1 illustrates the positioning of a discharge grate in a ball mill. The mill comprises a shell 1 of general cylindrical shape having a liner 2. A feed inlet end 3 provides means for introducing pulp and balls into the interior of the ball mill and a discharge outlet 4 discharges ground pulp. The mill is rotated by means of a drive, not shown, connected to the drive gear 5 encircling one end of the shell 1. A grate, shown in general by numeral 6, is positioned near the discharge outlet 4 for passing ground pulp out of the mill and to retain the charge of balls and large particles therein. In the grate discharge type of ball mill a high circulating load of pulp is utilized and a wet or hydraulic classifier, not shown, is used with the majority of wet ball mills, wherein the material of fine particle size is separated and passed on through the process and the oversize material is passed back into the mill as the circulating load.

The grate, illustrated in detail in Fig. 2, comprises a series of grate sectors, numbered 8 through 15, which are mounted on a suitable supporting member to form a substantially annular grate member. The grate sectors maybe held in place by various nieans,'sorne of which are now in common-practicein the art;'that is, by separate wedge bars, as Sh'ownin U'.S. Patent 1,690,493 issued to F. 7E. Marcy or bolted through holes in the sector as shown in U.S. Patent 1,139,651 issued W. B. Easton. While the shape of the sectors has been shown as truncated triangles, they may be of substantially any shape; that is, triangular, rectangular, square, etc. The number of the sectors will, generally, vary with the diameter of the mill. Thegrates, as illustrated, are arranged so that the inner ends rest against a blank center casting 7. One-or more of the sectors or sections may have a plurality of slots 17,-indicated on sectors 8, 9, 11, 13 and 15. The'slots will be more fully described below. The slots 17 of Fig. 2, shown in detail on section 9, have their major .or transverse axes sloped at about 28 to tangential line 18 which is perpendicular to the mill radially bisecting the sector. The bisecting radial extends through the plane of symmetry of the section, and this tangential or perpendicular line 18 is the reference line for determining the transverse slope of the slots.

, The slope of the major axes of slots from a tangent or perpendicular to the bisecting radial will vary according to the ball load and themill speed. The angle may vary'from about 13 degrees, for example, for a mill operating with a fifty percent ball load, to a thirty-four degree angle for a twenty-five percent ball load. The same angles, however, may apply if the mill is operating with a different ball load, but at a speed to cause approximately the same parabolic path. One such path is indicated in Fig. 2 by dashed line 26. Furthermore, the direction of the slope in relation to the rotation must be considered; for example, as shown in Fig. 2 the slots 17 in sector 9 slope upwardly to the left of center of the mill with the indicated clockwise rotation. If the mill had counterclockwise rotation, however, the slots would have to slope to the right, as shown in Fig 5, solas to be approximately parallel to the line of fall of the balls, on the right side of the mill, as the sector approaches the horizontal line 21 of the mill.

As the slots of the sector approach the horizontal line, the openings on the wearing-face are in position to receive the greatest amount of pulp, and it is in the area near the top of'the active load that the greatest release of pulp takes place. At this position the angle 'of the passages through; the grates, are at a declining angle to the discharge which prevents any spill back into the mill and thus accelerates the discharge through the discharge opening.

The sector 23, illustrated in Figs. 3 and 4, has a series ofslots' extending throughthe sector. Each slot provides a tangentially-disposed opening or passage extending from'the wear face 24 through the discharge face 25. One of the upper slots, which is typical of all the slots through the grate, Will be described specifically with reference to Figs. 3 and 4. The slot has an opening 26 on the wear face 24 which is a narrow, elongated slot having rounded ends 29. A corresponding opening 27 on the discharge face 25 is slightly wider than the opening 26 so that a passage 28 through the sector is tapered from the wear face to the discharge face. Furthermore, the opening 27 is positioned closer to the inner edge 31 of the sector than the peripheral portion 30. The angle of the passage to the horizontal may extend up to about forty-five degrees so long as there is no straight, axially parallel passage through the grates; however, the preferred slope is from fifteen to thirty degrees with the optimum at about thirty degrees. The direction of rotation of a ball mill using a grate as illustrated in Fig. 3 maybe either clockwise or counterclockwise. As the balls cascade across the wearing face of the grates the wearing side of'the'angle is at its lowest point-in relation to the parabolic path of the zones of action. Any particles which have a tendency to remain in the slots and protrude from the wearing face are wiped or knocked out of the slots by the rubbing or blow action of the cascading balls and it is this effect that keeps the slots free of plugging or choking.

A sector 35, illustrated in Figs. 5 and 6, includes a pcripheral edge 36 and an inner edge 37 on the sector which is of substantial'thickness. Slots are extended through the sector, and since each of the slots is substantially the same, reference to a single slot will be utilized as representative of all the slots. Such a slot having an opening 38 on the wear face 39 of the sector 35 and includes a passage 41 extending to an enlarged opening 41 on the discharge side 42 of the sector. The slot 41 is placed nearer to the inner edge 37 than the slot 38 so that the passage 4i) slopes toward the center of the mill. The passage 49, also tapers from the opening 38 to the opening 41 so that any'material which passes the opening 38 will pass through the passage 40 and out the outlet 41. The angle'A at which the opening 33 slopes to the horiiontalor a tangential line is determined by the ball load and'the mill speed. The slot is arranged to be substantially parallel to the average flow line of the ball load, for example line 20 of Fig. 2, at about the horizontal level of the mill.

Some examples of the variation of angle A from the tangent, when the slot is in correct direction in relation to rotation of the mill and at a particular mill speed, of the slots is indicated below.

Slope of slots Ball charge: (from horizontal) By c'hanging the mill speed a different parabolic path is obtained with a constant ball load, so it is obvious the slope of the angle is dependent on the ball load and mill speed. i r 1 In using the grate in ball mills, where the angle of the passage through the grate is inclined toward the center of theball mill, it will be readily understood that the balls cascade across the wearing face of the grates along the lateral extent of the slots. As far as plugging is concerned, the toe of the path produces the most severe action,'about forty percent of the total plugging, and by having the passage angle sloped upwardly in the toe position pl ugging is'substantially reduced. In other words smalli balls and tramp steel do not readily enter the slot due to the upward angle of the passage. Near the top of the-parabolicpath where the discharge of pulp is the greatest the angle of the passage is horizontally outward and declining so that pulp may readily pass through. Since the plugging action at this position is at a minimum, the action of the balls on any small balls or tramp steel being forced into a slot is at a minimum even though there is maximum pulp flow.

While the invention has been illustrated by reference to-specific examples, there is no intent to limit the inventionto theprecise details so set forth, except insofar as defined inthe'following claims. claim:

a 1. A discharge grate for a grinding rnfll, comprising a plilrality of grate sections arranged as an annular assembly'for closing the discharge end of such a mill and held inla verticalposition for conjoint rotation, each said section'having a plurality of elongated slots inclining from itsinner wear face-through its outer discharge face so as to pesition the'dischargeerid of the slot nearerthe mill 1 s'thanits intake end, each said slot transversely of the section being naImWer-at the wear face than at the discharge face of the section, and the slots of said sections having their major axes inclining toward the periphery of the grate in the direction of mill rotation at an angle to the perpendicular to the bisecting radial of the sec tion not greater than 35.

2. A discharge grate for a grinding mill, comprising a plurality of grate sections arranged as an annular assembly for closing the discharge end of such a mill and held in a vertical position for conjoint rotation, each said section having a plurality of elongated slots inclining from the inner wear surface through its outer discharge face so as to position the discharge end of the slot nearer the mill axis than its intake end, the slots of said sections having their major axes inclining toward the periphery of the grate in the direction of mill rotation, at an angle to the perpendicular of the bisecting radial of the section of from 13 to 35 degrees.

3. A grate member adapted to be arranged With other similar members to form an annular discharge grate for a grinding mill, comprising a body of substantial thickness having means for its support in a substantially vertical position Within such a mill, there being a series of spaced, elongated slots in said body having their major axes sloped to the perpendicular of the bisecting radial of said member at an angle of not more than about and the slots through said body being inclined from an inner wear face to an outer discharge face of said body so as to position the discharge end of each slot nearer the mill axis than its intake When mounted in a mill.

References Cited in the file of this patent UNITED STATES PATENTS 1,126,085 Riisager Ian. 26, 1915 1,139,651 Easton May 18, 1915 1,602,434 Johnson Oct. 12, 1926 1,690,493 Marcy Nov. 6, 1928 FOREIGN PATENTS 39,066 Germany Apr. 21, 1887 107,006 Sweden Jan. 28, 1943 152,724 Great Britain Oct. 8, 1920 239,988 Germany Oct. 28, 1911 

